In phototypesetting machines, it is often desirable to drive the film in both a forward and a reverse direction within the imaging plane, without causing buckling of the film, which buckling degrades the image projected upon the film. For single direction operation, the transport device disclosed in U.S. Pat. No. 4,046,372 to Peter Ebner is satisfactory. However, should this device be employed for bi-directional film transport, the film will buckle when being drive in the reverse direction by the single pair of drive rollers especially if an encoder is used to measure the travel for corrective feedback, and if this encoder actually is driven by the medium. U.S. Pat. No. 3,411,584 employs an active tensioning system utilizing an auxiliary motor that is connected to a tension roller by a slip coupling. During feed cycles, the frictional force generated by the moving sheet material overcomes the slip coupling to rotate the tension roller. Conversely, during static periods, the auxiliary motor drives the tension roller in a reverse direction to maintain tension on the stationary sheet. Although maintaining tension during both static and dynamic periods, the system disclosed in this patent exhibits the disadvantage of producing continuous rotation of the tension drive mechanism. For bi-directional operation, a second pair of drive rollers have been employed in the prior art to maintain the proper tension in both the forward and reverse direction to prevent buckling.
In a co-pending application of Alfred Ouelette, Ser. No. 159,428 filed June 16, 1980 now U.S. Pat. No. 4,351,460, two pairs of drive rollers are employed for bi-directional operation along with a relatively complex mechanism which employs clutches to obtain the differential speed necessary to keep the paper in tension for proper focusing. While this solenoid controlled paper transport is satisfactory for newspaper applications, it is desirable to provide a bi-directional transport useful in connection with more demanding commercial applications and yet is extremely simple to manufacture, and thus, more reliable and less costly.